There are two Hebrew words of the "Old" Testament (not an entirely accurate term e.g. see The Older Testament; also Christ In The Psalms and First And Second Comings In Isaiah - most "Old" Testament Prophecy hasn't happened yet) that are translated in English Bibles as "blaspheme." The first, pronounced naw-ats, means to scorn; the other, pronounced baw-rock, means to curse in a treasonous manner.
In the New Testament, the Greek word pronounced blas-fay-me-oh (from which the English word blasphemy itself originated) means to vilify i.e. to spread antagonistic or hateful information about someone - to slander.
In whatever language, blasphemy is the mark of an evil fool.
"10:8 The wise in heart will receive commandments: but a prating fool shall fall." (Proverbs 10:8 KJV)
"A blasphemer, and a persecutor, and injurious"
"24:15 And thou shalt speak unto the children of Israel, saying, Whosoever curseth his God shall bear his sin. 24:16 And he that blasphemeth the name of the LORD, he shall surely be put to death, and all the congregation shall certainly stone him [see also Stoning]: as well the stranger, as he that is born in the land, when he blasphemeth the name of the LORD, shall be put to death." (Leviticus 24:15-16 KJV)
Evil behavior, by people who claim to be God's people, fuels blasphemy of God by those who reject Him in name and deed (as opposed to those who reject Him in deed, while claiming His name - see Face Value).
"12:14 Howbeit, because by this deed thou hast given great occasion to the enemies of the LORD to blaspheme..." (2 Samuel 12:14 KJV)
The apostle Paul (see Paul's Ministry) referred to the principles of Isaiah 52:5 ("52:5 Now therefore, what have I here, saith the LORD, that my people is taken away for nought? they that rule over them make them to howl, saith the LORD; and my name continually every day is blasphemed") and Ezekiel 36:22 ("36:22 Therefore say unto the house of Israel, Thus saith the Lord GOD; I do not this for your sakes, O house of Israel, but for mine holy name's sake, which ye have profaned among the heathen, whither ye went") in how false and/or corrupt "Christians" cause God to be blasphemed by "Gentiles":
"2:21 Thou therefore which teachest another, teachest thou not thyself? thou that preachest a man should not steal, dost thou steal? 2:22 Thou that sayest a man should not commit adultery, dost thou commit adultery? thou that abhorrest idols, dost thou commit sacrilege? 2:23 Thou that makest thy boast of the law, through breaking the law dishonourest thou God? 2:24 For the name of God is blasphemed among the Gentiles through you, as it is written." (Romans 2:21-24 KJV)
Paul by that time had ceased being a blasphemer (see Paul's Blindness Lesson) - and was forgiven because he "did it ignorantly in unbelief."
"1:12 And I thank Christ Jesus our Lord, who hath enabled me, for that he counted me faithful, putting me into the ministry; 1:13 Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief." (1 Timothy 1:12-13 KJV)
Others who committed (and commit) blasphemy are not all as innocent as Paul was (see The Unpardonable Sin).
"12:24 But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils [see Baal-zebub and Beelzebub].
In the end-time just prior to Christ's return, blasphemy in the world will reach its peak, led by two Satan-empowered people. The first, a great military/political "beast" power (see Ein Volk! Ein Reich! Ein Euro!):
"13:1 And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy. 13:2 And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon [see Dragons] gave him his power, and his seat, and great authority [see Daniel's Dream Of Four Beasts]. 13:3 And I saw one of his heads as it were wounded to death; and his deadly wound was healed: and all the world wondered after the beast. 13:4 And they worshipped the dragon which gave power unto the beast: and they worshipped the beast, saying, Who is like unto the beast? who is able to make war with him?
The other will be the Beast's "great false prophet" (see Emperors and Popes).
"17:1 And there came one of the seven angels which had the seven vials, and talked with me, saying unto me, Come hither; I will show unto thee the judgment of the great whore that sitteth upon many waters: 17:2 With whom the kings of the earth have committed fornication, and the inhabitants of the earth have been made drunk with the wine of her fornication.
Fact Finder: What is it that makes the devil "Satan" - his actions, or his evil mouth?